Epoxy resins are monomers or prepolymers that react with curing agents to yield high-performance resins. These resins have gained wide acceptance in protective coatings, electrical insulation, structural adhesives, and in structural applications as a matrix resin for composites because of their combination of characteristics such as thermal and chemical resistance, adhesion and abrasion resistance.
Epoxy resins are characterized by the presence of a 3-member cyclic ether group commonly referred to as an epoxy, 1,2-epoxide or an oxirane group. The epoxy resins are cured, or caused to harden, by the addition of a curing or hardening agent. Curing agents used include anhydrides, amines, polyamides, Lewis acids, salts and others. The most common class of epoxy resins are diglycidyl ethers that are cured by the use of polyamino compounds.
Epoxy resins are frequently required to have high glass transition temperatures in order to have structural properties at high temperatures. A method of achieving high glass transition temperatures in epoxy resins is to prepare resins having high crosslink density and a high concentration of polar groups. This technique is disclosed in U.S. Pat. No. 4,331,582 where it is taught that bis[4-(N,N-diglycidylamino)phenyl]methane (TGDDM) is cured with di(4-aminophenyl)sulfone (DDS). While this method does produce resins that have high glass transition temperatures, the resins have several shortcomings. The materials are very brittle and suffer a large loss in glass transition temperature when exposed to moisture. These problems are caused by the high crosslink density and high concentration of polar groups respectively. DDS can also be used to cure other epoxy resins such as glycidyl ethers of polyhydric phenols. Again while these resins may be characterized by high glass transition temperatures, they also tend to be very brittle.
Epoxy resin compositions in which the epoxy group-containing compound contains a polycyclic structure are known that can be cured to resins having a high glass transition temperature. Examples of such resin compositions among others are the glycidyl ethers of polyhydroxy-phenylchroman disclosed in U.S. Pat. No. 2,902,471 and the bisglycidyl ethers of polycarbocyclic substituted bisphenols, e.g., (2-norcamphanylidene)diphenol, described in U.S. Pat. No. 3,298,998; the bisglycidyl ethers of cyclopentenyl substituted bisphenols disclosed in U.S. Pat. No. 3,332,908. Although these compositions can be cured to resins having a high glass transition temperature, the cured resins are highly crosslinked and have low ductility and, compared to the cured resins of the instant invention, have a relatively high water absorption.
Also known are epoxy resin compositions, in which the curing agent or hardener contains a polycyclic structure, that can be cured to resins having a high glass transition temperature. Examples of such resin compositions are those in which the curing agent is an aromatic (can be diphenyl) polyamine in which the amino groups are primary or secondary such as is disclosed in U.S. Pat. No. 3,397,177, a dicyclohexyl polyamine as is described in U.S. Pat. No. 3,963,667; a phenylindane diamine such as is disclosed in U.S. Pat. No. 3,983,092; the tricyclic and pentacyclic polyamines having 2 to 4 primary and 1 or 2 secondary aminoalkyl groups as are described in U.S. Pat. No. 4,229,376; and the cycloaliphatic polyamines described in U.S. Pat. No. 4,321,354. Although many of these compositions can be cured to resins having a high glass transition temperature, the cured resins are highly crosslinked, have low ductility, and, compared to the cured resins of the instant invention, have a short shelf life and may have a relatively high water absorption.
Thermoplastic resins having a high glass transition temperature, i.e. above about 120.degree. C., are known to be obtainable by incorporating into the resin an internal or pendent polycyclic structure. Examples of such resins are the polyamides disclosed in U.S. Pat. No. 3,143,530 and in U.S. Pat. No. 3,287,321; polyimides described in U.S. Pat. Nos. 4,358,582 and 4,366,304; the polyesters of U.S. Pat. Nos. 3,546,165 and 4,388,455, among many other patents. Other resins having internal or pendent polycyclic structure are discussed by Korshak et al., J. Macromol. Sci.--Rev. Macromol. Chem., C11(1), 54 (1974).
It is seen from the above that many epoxy compositions can be cured to resins having a high glass transition temperature. Generally, this has been done by use of a curing agent that brings about a high degree of crosslinking of the epoxy composition which has led to a corresponding increase in brittleness of the resultant cured resin. It is believed that few if any curing agents in the prior art provide cured resins having a combination of high glass transition temperature, high ductility and low moisture pick-up. The resins of the prior art have either high glass transition temperature and relatively low ductility by reason of a high degree of crosslinking or high ductility and relatively low glass transition temperatures by reason of low crosslinking of the cured resin. Further, it is known in the art that epoxy resins compositions which are cured resins that have a high glass transition temperature and high moisture pick-up will generally undergo a reduction of glass transition temperature and strength upon absorption of moisture.